The present invention relates to a method and apparatus for generating seismic waves, and in particular a method and apparatus for generating seismic waves to improve reservoir performance and well output.
The use of vibratory seismic wave sources of various types for stimulating oil reservoirs is known.
For example, explosion-type sources positioned within a well can be used to stimulate a reservoir. The sources may comprise charges, such as tanks containing liquid or gas fuel materials and priming cartridges with powder and solid explosives. Another known method involves preparing an explosive mixture directly in a well by water electrolysis [1-5].
However, problems associated with such methods include: the low power of the sources; the inconvenience of lifting an apparatus to the surface to equip it for subsequent stimulations; and the risk of accidental explosion of components of the apparatus. The risk of damaging the well prevents the user from increasing the amplitude of the vibratory stimulation by applying more powerful solid or liquid blasting substances.
Mechanical stimulation of reservoirs [6,7] involving periodically applied impacts against the well bottom may also be used to improve oil production. These methods, however, suffer the following disadvantages:
1. They are technically complex because the stimulation is effected by the impact of a long string (about 1 km) placed inside a well; and
2. As the time of impact is quite long (about 1 s), the rock is only locally affected. Furthermore the longitudinal waves transmitted into the rock mass have a very small stress amplitude.
Therefore, in general, known methods of vibratory stimulation of oil reservoirs using sources positioned within a well suffer from a restricted stimulation area and are efficient only at the bottomhole zone of the reservoir due to the small stimulation amplitude.
An alternative method of wave stimulation of a deposit is taught in [8]. The method is based on the stimulation of a rock mass by a powerful vibrator which rests on the surface and has a vibration frequency matching the frequency of characteristic vibrations of the producing formation. In contrast to the abovementioned methods, the stimulation area of this method is not restricted to the bottomhole zone of the formation and may cover a considerable area of the deposit.
However, this alternative method suffers from a low efficiency due to high attenuation as the waves progress from the surface, especially through the weathering zone (i.e the surface geological layer which is affected by atmospheric weathering). For this reason, to accumulate significant residual strains (which increase formation permeability and thereby improve reservoir performance) in a reservoir bed, a large number of stimulations must be performed.
In the separate field of seismic exploration, known apparatuses for generating seismic waves comprise gas mixture preparation units and operate by the explosive energy of such mixtures.
Seismic sources comprising an explosion chamber located on the surface tend to be inefficient because the waves attenuate as they dissipate from the surface, especially through the weathering zone [9].
One known seismic wave generation apparatus employed in seismic exploration comprises [10] a pipe, open at its lower end, which is adapted to be driven into the top layers of soft earth. Oxygen and acetylene are supplied by a valve system, mixed in a predetermined ratio, and introduced into the inside of the pipe to fill the pipe and voids in the earth in the vicinity of the pipe lower end. The gas mixture is ignited at the top end of the pipe and the detonation wave progresses down the pipe to detonate the gas filling the voids near the lower section of the pipe. After several successive explosions a void space of a constant volume is formed in the earth. Seismic waves are transmitted into the earth as the result of detonating the mixture. The process is repeated after a predetermined time interval.
However, this apparatus suffers the following problems:
1. Its efficiency is low due to attenuation as the waves progress through the weathering zone;
2. It cannot be employed in hard rock;
3. The use of oxygen and acetylene leads to a high risk of accidental explosion;
4. Local conditions affect the stability of the source; and
5. The transmitted transverse waves are of low intensity.
Thus it is an object of the present invention to improve hydrocarbon (e.g. oil or gas) recovery from reservoirs and well output. A further object of the present invention to improve the efficiency of transmission of transverse waves from a seismic source in dense rock, and to enable repetitive transmission of waves at predetermined time intervals.
In a first aspect the present invention provides a method of generating seismic waves in a rock mass surrounding a well, the method comprising the steps of: (a) providing a packer and a fluid withdrawal line within the well, wherein the packer is located below the weathering zone and defines the upper limit of a working volume within the well, and the fluid withdrawal line is adapted to withdraw fluid from the working volume; and (b) providing and detonating an air-fuel mixture in the working volume to generate seismic waves, combustion products of the detonation being withdrawn from the working volume via the fluid withdrawal line.
The fuel of the air-fuel mixture is typically a hydrocarbon, for example a hydrocarbon gas such as propane or methane. Conveniently, the air of the air-fuel mixture is the surrounding, oxygen-nitrogen atmosphere. However, the air may be any, typically oxygen-based, naturally-occurring or artificial gas or gas mixture which can react with the fuel in an explosive manner.
The method can be used to stimulate hydrocarbon production from a hydrocarbon well. As the air-fuel mixture is deployed downhole, the risk of an accidental surface explosion can be reduced. Furthermore, by evacuating the combustion products from the working volume, the method can be repeated at relatively short time intervals. Indeed, in a preferred embodiment, step (b) is performed repeatedly.
Thus the present invention can enable long-term stimulation of hydrocarbon reservoirs by high-intensity transverse waves caused by repetitive detonations of a gaseous air-fuel mixture within a well working volume of predetermined length located in dense rock below the weathering zone.
A further advantage of the method is that the force of the detonation can easily be controlled in response to below the weathering zone, the seismic waves are still relatively strong when they reach the target formation.
Preferably, the method further comprising the step of filling the working volume with air before step (b). In this way any liquid (e.g. water, mud etc.) in the working volume can be evacuated before the air-fuel mixture is provided.
In one embodiment, step (a) further comprises providing a detonation transmission line which terminates at one end in the working volume, and step (b) further comprises causing a priming detonation at the other end of the transmission line to detonate the air-fuel mixture in the working volume. This allows the working volume detonation to be controlled remotely, typically from the surface. An electric spark discharge may be used to initiate the priming detonation.
Preferably, the detonation transmission line expands out in a cone-shape at its terminus in the working volume. The diameter of the detonation transmission line is typically significantly smaller than that of the working volume, but the cone-shape encourages the detonation wave which travels along the detonation transmission line to propagate smoothly into the working volume.
Conveniently, the air for the air-fuel mixture may be supplied to the working volume via the detonation transmission line, thereby avoiding the need to provide a separate air supply line.
Preferably the fuel for the air-fuel mixture is supplied to the working volume by a separate fuel supply line. By keeping the fuel and air separate until they are mixed in the working volume, the risk of accidental explosions is further reduced.
The fuel may be supplied to the working volume at spaced positions along the working volume, thereby improving the mixing of the fuel and air in the working volume.
In a further aspect, the present invention provides a method for generating seismic waves in a rock mass surrounding a well, comprising the steps of: (a) installing a packer and a drain pipe within the well, and supplying air and fuel inside the well, said packer being located below the weathering zone; (b) filling a working volume of the well with air, said working volume being a well region between the packer and a lower end of the drain pipe; and (c) repetitively detonating an air-fuel mixture within said working volume.
The method may be used to effect wave stimulation of an oil reservoir.
Preferably, the method further comprises supplying air into the working volume through a detonation transmission line.
Preferably the volume of fuel supplied into the working volume equals the sum of the volumes of the detonation transmission line and the working volume of the well.
In a further aspect, the present invention provides a method of stimulating an oil reservoir of a well, the method including repetitively generating high-intensity transverse seismic waves transmitted as the result of the detonation of a gas air-fuel mixture within a working volume of the well, the working volume being defined by the distance from a packer to the lower end of a drain pipe and being located in dense rock below the weathering zone.
Optional features of the method of any one of the previous aspects may be applied to the method of any other of the previous aspects.
In a further aspect, the present invention provides an apparatus for generating seismic waves in a rock mass surrounding a well, the apparatus comprising: (a) a packer which is locatable in the well below the weathering zone to define the upper limit of a working volume within the well; (b) a fuel line which is locatable in the well to supply the fuel for an air-fuel mixture in the working volume; (c) a detonation transmission line which is locatable in the well to terminate at one end in the working volume whereby a priming detonation at the other end of the line causes the air-fuel mixture in the working volume to detonate; and (d) a fluid withdrawal line which is locatable in the well to withdraw fluid from the working volume, whereby combustion products of the detonation of the air-fuel mixture are withdrawable via the fluid withdrawal line.
Thus the apparatus can be used to detonate an air-gas mixture within a region of well drilled into dense rock and below the weathering zone, whereby high-frequency pulses of substantial amplitude can be transmitted throughout the rock mass.
The detonation transmission line may be adapted to supply the air for the air-fuel mixture in the working volume. Furthermore, the detonation transmission line may expand out in a cone-shape at its terminus in the working volume. Preferably, the detonation transmission line comprises a priming unit for causing the priming detonation, and more preferably the priming unit is an electric spark discharge generator.
Preferably, the packer is adapted to allow the fuel line, the detonation transmission line and the fluid withdrawal line to pass through the packer to the working volume.
The fuel line may have spaced perforations to supply the fuel at spaced positions along the working volume. Preferably, the perforations are uniformly spaced.
In one embodiment, the apparatus further comprises a fuel reservoir which is connectable to the fluid line whereby the fluid line supplies fuel to the working volume from the reservoir, the amount of fuel holdable by the reservoir being variable so that a stochiometric air-fuel mixture is providable in the working volume.
In a further aspect, the present invention provides an apparatus of the previous aspect which is installed in a well. Preferably, the well is a hydrocarbon well.
In a further aspect, the present invention provides an apparatus having: a packer, a drain pipe, a detonation transmission line and a fuel line, said drain pipe, detonation transmission line and fuel line passing inside the well, wherein said packer is located below a weathering zone, said detonation transmission line is adapted to fill a working volume of the well with air, and the working volume is a well region between the packer and the lower end of the drain pipe.
The fuel line may comprise a fuel receiver or reservoir having a variable volume. Preferably the volume of the fuel receiver or reservoir equals the sum of the volumes of the detonation transmission line and the working volume.
In a further aspect, the present invention provides an apparatus comprising a packer with a drain pipe and a valve, a detonation transmission line to transmit detonation into a well working volume, and a filling system (comprising e.g. a control panel) for filling the working volume with a gas mixture, the working volume being bounded by the packer above and by (e.g. a lower end of) the drain pipe below. Preferably the working volume from through the detonation progresses is a well region located in dense rock below the weathering zone and bounded by the packer and the lower end of the drain pipe.
Optional features of the apparatus of any one of the previous aspects may be applied to the apparatus of any other of the previous aspects.